1. Technical Field
The invention relates to a method for producing hollow, composite construction elements, preferably intended for use in vehicles, and a construction element produced according to the method.
2. Background Art
In many fields, there is currently a demand for weight-optimized products with maintained functioning and strength. This applies especially to forged products, which can be heavy and difficult to optimize because of limitations inherent to the tools used in their production.
One example is front axle beams for heavy vehicles. These beams are typically forged as an I-profile in which the web, or core of the beam cross section has little effect upon the torsional rigidity. Using strength calculations, it can be shown that a tubular cross section, with the material moved as far radially outwards as possible, is best for such a construction. This applies especially to the portion that is commonly referred to as the “swan neck” in the front axle beam which is located between a central part and a spindle holder of the beam. With traditional forging methods, it is difficult to achieve such a solution. European publication EP-A2-0 059 038 shows a front axle beam forged horizontally in the traditional manner; that is to say that the blank lies with its final vertical plane (after being fitted) in the horizontal plane as it is worked. Therein, it is described how a blank is pre-formed by means of rolling and is subsequently moved between a number of presses which forge the whole or parts of the blank into the desired shape. As previously stated, the drawback with the solution is that the web of the beam, for the greater part, is centrally located, which has little effect upon the torsional rigidity.
An alternative solution can be seen from European publication EP-A1-0 015 648 which describes the forging of a rectangular, hollow front axle beam starting from a tubular blank. Although it is possible with this method to obtain a beam with higher torsional rigidity, it also creates a number of problems. In order to produce tapered ends of the beam, the tubular blank has to be drawn through a die. Even if the material is distributed radially, viewed further out from the center of the beam, the possibility of controlling the material thickness is very limited. This also applies to the other parts of the beam, since the starting material is a tube of constant material thickness. In addition, a great deal of working of the ends of the beam is required, in order to produce spindle holders, and fitting of separate fixtures for such things as air bellows.
A further solution can be seen from U.S. Pat. No. 6,122,948 that shows a hydro-formed front axle beam. In this case too, the starting point is a tubular blank, which is first bent into the desired basic shape and is subsequently hydro-formed formed into its final shape. One drawback with this solution is, like the example above, that the distribution of the material along the length of the profile cannot be controlled. The profile must also be provided with a plurality of separate fastenings, not only for the air bellows but also fixtures for the steering spindle bolts. The latter have to be fastened by, for example, welding which gives the beam a natural weakening susceptible to corrosion.
Finally, it is also possible to cast hollow front axle beams, as can be seen from JP-A-11-011105. For technical reasons of casting practice, there are restrictions, however, in terms of maximum and minimum material thickness, as well as the need for strengthening ribs, complicated casting cores and the like in order to allow a sufficiently advanced profile to be cast. In addition to this, there are further restrictions in terms of a practical choice of material, as well as financial implications for the unit price of the axle beams owing to the heavily increased costs that a casting process entails.
The majority of the abovementioned problems are solved by the production method conducted according to the present invention that is disclosed herein because the method offers a substantially greater chance of precisely controlling the distribution of material around and along a forged profile.